Fail-safe electromagnetically operated brake devices have been found to be particularly desirable in applications where an electrical power failure is a possibility. In those applications it is desirable, upon loss of electrical power, to engage the brake in order to bring a device to a complete standstill condition and maintain the standstill condition until power has been restored. For example, in an electric lift truck, it would be desirable, in the case of an electrical system failure, to stop the forks in the position that they attained prior to the electrical failure. This would insure that the lift with its heavy load would not drop to the floor and cause serious damage. Also, in a lift truck, the fail-safe electromagnetic brake can be used when the engine stalls so the truck would come to a complete standstill condition upon power failure.
Other uses of this type of device are in an elevator where it would be necessary to stop the elevator in case of a power failure. Thus, in general, the fail-safe electromagnetic brake is used in any application where it is desired, in cases of a loss of electrical power, to stop functioning of certain electrical components to avoid a major breakdown or disaster.
Such applications place particular requirements on the brake device in that generally the brake device must be simple and economically constructed and at the same time provide positive engagement under a high torque absorption in order to enable the mechanical system in which the brake is utilized to safely avoid continuation of the event cycle. A brake in such a system may be either off or on for long periods of time or cycled on and off for short periods of time depending on the application requirements. Thus, the brake must be sufficiently durable to withstand constant use with a minimum amount of wear.
One prior art design utilizing an electromagnetic clutch or brake with a self-adjusting feature is shown in Miller et al, U.S. Pat. No. 3,994,379, owned by the assignee of the present application. In this embodiment, the friction member is threadably engaged to an armature with relative motion in one direction prevented between the armature and the friction member by a self-adjusting retarder. The fingers of the retarder act upon a knurled surface on the armature to prevent relative motion in one direction between the armature and the friction member on disengaging of the electromagnetic coil. The retarder member only permits the armature to rotate in one direction relative to the friction member, that is, as the frictional surface wears away. As wear occurs, and the electromagnetic coil is energized, the rotating armature and friction member are moved axially towards the pole faces. The rotational velocity of the armature is decreased by the static condition of the pole faces. The friction cone member, which continues to rotate at the input shaft speed, is caused to move axially forward along the mutually engageable thread between the friction member and the armature, that is, towards the brake, thereby advancing the friction surface of the friction member into engagement with its mating surface to apply the brake.
This self-adjusting brake device is, however, complex, difficult to assemble and expensive to manufacture. Further, it has been found that it is difficult to make and expensive because of the many manufacturing steps required to form the retarder member. Furthermore, this device, while self-adjusting, is not fail-safe.
A further improvement to the aformentioned design is shown in Miller, U.S. Pat. No. 4,030,583, also owned by the assignee of the present application. U.S. Pat. No. 4,030,583 teaches a fail-safe electromagnetic cone clutch or brake device for transmitting torque from an input means to an output means. The input means includes a clutch actuating mechanism including an electromagnetic winding, an armature coaxially disposed with the input means and threadably connected to a driving friction ring member. The driving friction ring member is splined to move axially on an inner body member which is mounted to the input shaft. When the clutch is de-energized, biasing springs move the friction cone ring member against the output means thus transmitting torque from the friction ring to the output means. When the electromagnetic winding is energized, a circular flux path is generated between the inner body member and the armature located adjacent to the inner body member. The armature is threadably connected to the friction ring member. When the electromagnetic winding is energized, the armature pulls the friction ring member away from the output means thereby stopping the transmittal of torque from the input means to the output means. The friction device maintains this position until the electromagnetic winding is again de-energized. A retarder member is disposed adjacent to the armature to permit engagement of the friction ring member to the output means without adverse wear on the armature. The retarder member has a plurality of tabs on its outer diameter corresponding to a plurality of grooves on the inner surface of the opposite end portion of the friction ring member.
This fail-safe self-adjusting brake is, however, also complex, difficult to assemble and expensive to manufacture. In particular, knurling the opposite face of the armature member has proven to be difficult in production. In addition, the retarder member with its plurality of tabs and spring-like fingers are expensive to make. Further, this device permits excessive overadjustment of the armature member relative to the friction member.
Therefore, none of the aforementioned designs provides a simple, reliable, self-adjusting, fail-safe electromagnetic brake which prevents excessive overadjustment during the wear compensating mode and provides a retarder member which is expensive and simple to make.